Currently, radiation sources which are energy-efficient, high-intensity, and/or provide a high power density, such as high-power LEDs (light-emitting diode), lasers, for example, in the form of laser diodes, and/or superluminescent diodes are increasingly being used in modern illumination units. In contrast to incandescent bulbs, which are thermal radiators, these radiation sources emit light in a narrowly limited spectral range, so that the light thereof is nearly monochromatic or exactly monochromatic. One possibility for opening up further spectral ranges is, for example, radiation conversion, in which phosphors are irradiated by means of LEDs and/or laser diodes and in turn emit radiation of another wavelength. In so-called “remote phosphor” applications, for example, a phosphor-containing layer located at a distance from a radiation source is typically lighted by means of LEDs or laser diodes and in turn emits radiation of another wavelength. For example, this technology can be used in order to convert the light of blue LEDs by admixing yellow light, which is generated by exciting a phosphor-containing layer, into white light.
Furthermore, projectors are regularly currently used to optically display data. Such a projector projects the data to be displayed in the form of individual stationary and/or moving images on a projection screen, for example. Generating the required excitation radiation with the aid of a conventional discharge lamp, i.e., an ultra high pressure mercury vapor lamp, for example, is known in the case of a conventional projector. More recently, however, LARP (laser activated remote phosphor) technology has also already been used. In this technology, a conversion element arranged spaced apart from the radiation source, which has phosphor or consists thereof, is irradiated using excitation radiation, in particular an excitation beam (pump beam, pump laser beam). The excitation radiation of the excitation beam is entirely or partially absorbed by the phosphor and converted into a conversion radiation (emission radiation), the wavelengths and therefore spectral properties and/or color of which are determined by the conversion properties of the phosphor. In the case of down conversion, the excitation radiation of the radiation source is converted by the irradiated phosphor into conversion radiation having longer wavelengths than those of the excitation radiation. For example, with the aid of the conversion element, blue excitation radiation (blue laser light) can thus be converted into red or green conversion radiation (conversion light, illumination light).
The excitation radiation can introduce a large amount of energy into the conversion element, whereby it can heat up strongly. This can result in damage to the conversion element and/or the phosphors contained therein, which can be provided as a single phosphor or a phosphor mixture. In addition, in the event of a lack of cooling of the phosphor, conversion losses arise as a result of efficiency reduction caused by thermal quenching. To avoid excessively strong heating and to avoid the possible damage linked thereto to the conversion element or the phosphor, arranging multiple conversion elements on a phosphor wheel (often also referred to as a pump wheel or color wheel), which is irradiated using the excitation beam while it rotates, is known. Because of the rotation, different conversion elements and/or regions of the conversion elements are successively illuminated and therefore the introduced light energy is distributed over the surface area.
Heretofore, a degree of miniaturization in LARP technology has been conceptually restricted, since the assembly which has the radiation source (pump laser) and the phosphor wheel requires a large amount of installation space. However, a smaller installation space is desirable for different applications, for example, in the field of pico-projection, i.e., in the case of small-dimensioned mobile projectors, and/or of miniaturized projection units in so-called embedded projection, in which the projection unit is integrated in a mobile telephone or a camera, for example. The thermal attachment of the conversion element is important in this case to avoid overheating and damage.
For remote phosphor applications, thin phosphor layers such as cubic silicate minerals, orthosilicates, garnets, or nitrides are applied to surfaces of corresponding carriers. The phosphor layers are normally mechanically fixed using binding agents on a carrier and attached to an optical system (lenses, collimators, etc.), wherein the light coupling can occur via air or by means of an immersion medium, for example. To ensure the most optimal possible optical connection of the optical system to the phosphor and avoid light losses, the most direct possible optical connection should be ensured. For remote phosphor applications, i.e., applications in which phosphor and radiation source, for example, high-power laser diodes, are spatially separated, for example, a thin phosphor layer is applied to a surface, for example, a substrate and/or a carrier, mechanically fixed using binding agents, and attached (air, immersion, etc.) to an optical system (lenses, collimators, etc.).
In the above-mentioned applications, the phosphors are excited to emission as is typical by means of LEDs and/or laser diodes using high light powers. The thermal losses arising in this case are to be dissipated, via the carrier, for example, to avoid overheating and therefore thermally-related changes of the optical properties or also the destruction of the phosphor. The phosphors are excited to emission, for example, using light sources of high power density (several W/mm2). The high thermal losses (Stokes) arising in this case result in an introduction of heat into the phosphor layer. If these temperatures become excessively high, for example, due to inadequate heat dissipation, thermally-related changes of the optical properties (emission wavelength, conversion efficiency, etc.) can occur, or finally the phosphors or the layer itself can be destroyed. Both phosphor and also binding agent can be the cause of this degeneration of the phosphor layer. For this reason, the phosphor layer is to be designed so that it can have optimum heat dissipation, to avoid the thermal destruction of the phosphors and the binding agent.
The phosphors, which are usually provided in powder form, do not form mechanically stable layers, i.e., abrasion-resistant and/or scratch-resistant layers, without an additional use of binding agents, for example, silicones. Binding agents are also used in general, however, to bring together the phosphor particles to form a phase, which can then be applied to corresponding surfaces. If binding agents are used for the layer stabilization, however, these binders themselves can interact with the phosphors and therefore negatively influence their optical and thermal properties, and also their service life. In addition, the thermal conductivity of the binding agents frequently represents a limiting variable in the dissipation of heat arising in the conversion element. Moreover, the binding agents are themselves to be thermally and spectrally stable and are to display little to no aging properties. For this reason, the use of an inert, optically transparent, thermally and spectrally stable binding agent is advantageous for the production of stable and longer-lived phosphor layers.
Using silicones as binder matrices for a light-technology excitation (for example, LEDs) is known. However, these do not permit excessively high light powers (power densities of several W/mm2) or make further technological expenditure necessary (for example, color wheels to reduce the light action time). The known phosphor-silicone mixtures are typically applied directly to metallic substrates. For example, the phosphor is suspended in organic matrices, for example, silicone, and then screen printed, for example. The layers are approximately 30 μm thick, for example. Silicone has a poor thermal conductivity of 0.1-0.2 W/m·K, which has the result that the phosphor heats up more strongly in operation and thus becomes more inefficient. This is problematic in particular in high-performance LEDs and in laser applications.
The coating process during the formation of a phosphor layer is limited by the type of the substrate materials. Thus, high-temperature processes are not conceivable on many plastics and metallic materials (for example, aluminum) because of their melting temperatures or heat resistance. Alternatively available ceramic materials having good thermal conductivity (for example, AlN), in contrast, are linked to increased technological and financial expenditure.
Inorganic matrices having improved heat dissipation are known from various publications, for example, low-melting-point glass from WO 2011/104364 A1 or metal phosphates from WO 2011/138169 A1.
Inorganic matrices have the disadvantage in relation to organic matrices, however, that to achieve a compact, bubble-free layer, relatively high temperatures are generally needed if a specific chemical stability (for example, in relation to UV radiation and/or moisture) is required. Typical softening temperatures of common low-melting-point glasses are from 500° C. to 600° C. At these temperatures, opto-electronic substrates, for example, an LED chip or substrates having good reflection, for example, highly-reflective aluminum or the phosphor to be embedded, in particular nitrides, are already damaged and thus become less efficient.
As alternatives, conversion elements are known, which are formed from a ceramic including the phosphor or from a crystal including the phosphor. In particular, the phosphor can form the ceramic or the crystal. Such conversion elements can be glued onto cooling bodies, so that the heat arising therein can be dissipated. A limiting variable for the dissipation of the heat is in this case the thermal conductivity of the adhesive used. Furthermore, it is favorable for good heat dissipation if the conversion elements are implemented as particularly thin.